12_a_1: Add solution

1 files changed, 91 insertions, 0 deletions

diff --git a/ch12/12_a_1.hs b/ch12/12_a_1.hs
new file mode 100644
index 0000000..2ab71b1
--- /dev/null
+++ b/ch12/12_a_1.hs
@@ -0,0 +1,91 @@
+-- 1. Write a function that takes two arguments: a four-element tuple and an
+--    integer. With an integer argument of zero, it should return the leftmost
+--    element of the tuple. With an argument of one, it should return the next
+--    element. And so on. What restrictions do you have to put on the types of
+--    the arguments in order to write a function that typechecks correctly?
+--
+-- 2. Write a similar function that takes a six-tuple as its first argument.
+--
+-- 3. Try refactoring the two functions to share any common code you can
+--    identify. How much shared code are you able to you find?
+
+-- In order for this function to typecheck correctly, all elements in the tuple
+-- must have the same type, so the return value's type is always the same for
+-- any value of the index.
+get4 :: (a, a, a, a) -> Int -> Maybe a
+get4 (a, _, _, _) 0 = Just a
+get4 (_, b, _, _) 1 = Just b
+get4 (_, _, c, _) 2 = Just c
+get4 (_, _, _, d) 3 = Just d
+get4 _ _ = Nothing
+
+get6 :: (a, a, a, a, a, a) -> Int -> Maybe a
+get6 (a, _, _, _, _, _) 0 = Just a
+get6 (_, b, _, _, _, _) 1 = Just b
+get6 (_, _, c, _, _, _) 2 = Just c
+get6 (_, _, _, d, _, _) 3 = Just d
+get6 (_, _, _, _, e, _) 4 = Just e
+get6 (_, _, _, _, _, f) 5 = Just f
+get6 _ _ = Nothing
+
+-- Refactorring the two functions to share common code
+get2 :: (a, a) -> Int -> Maybe a
+get2 (a, _) 0 = Just a
+get2 (_, b) 1 = Just b
+get2 _ _ = Nothing
+
+-- The 'get6' function can be refactorred to make use of the 'get4'
+-- function. However, this doesn't save much code.
+refGet6 :: (a, a, a, a, a, a) -> Int -> Maybe a
+refGet6 t i = maybe gy (\j -> Just j) gx
+  where (a, b, c, d, e, f) = t
+        x = (a, b, c, d)
+        y = (e, f)
+        gx = get4 x i
+        gy = get2 y (i - 4)
+
+
+-- ghci> :l 12_a_1.hs
+-- [1 of 1] Compiling Main             ( 12_a_1.hs, interpreted )
+-- Ok, one module loaded.
+
+-- ghci> get4 ('1', '2', '3', '4') 0
+-- Just '1'
+-- ghci> get4 ('1', '2', '3', '4') 1
+-- Just '2'
+-- ghci> get4 ('1', '2', '3', '4') 2
+-- Just '3'
+-- ghci> get4 ('1', '2', '3', '4') 3
+-- Just '4'
+-- ghci> get4 ('1', '2', '3', '4') 4
+-- Nothing
+
+-- ghci> get6 ('1', '2', '3', '4', '5', '6') 0
+-- Just '1'
+-- ghci> get6 ('1', '2', '3', '4', '5', '6') 1
+-- Just '2'
+-- ghci> get6 ('1', '2', '3', '4', '5', '6') 2
+-- Just '3'
+-- ghci> get6 ('1', '2', '3', '4', '5', '6') 3
+-- Just '4'
+-- ghci> get6 ('1', '2', '3', '4', '5', '6') 4
+-- Just '5'
+-- ghci> get6 ('1', '2', '3', '4', '5', '6') 5
+-- Just '6'
+-- ghci> get6 ('1', '2', '3', '4', '5', '6') 6
+-- Nothing
+
+-- ghci> refGet6 ('1', '2', '3', '4', '5', '6') 0
+-- Just '1'
+-- ghci> refGet6 ('1', '2', '3', '4', '5', '6') 1
+-- Just '2'
+-- ghci> refGet6 ('1', '2', '3', '4', '5', '6') 2
+-- Just '3'
+-- ghci> refGet6 ('1', '2', '3', '4', '5', '6') 3
+-- Just '4'
+-- ghci> refGet6 ('1', '2', '3', '4', '5', '6') 4
+-- Just '5'
+-- ghci> refGet6 ('1', '2', '3', '4', '5', '6') 5
+-- Just '6'
+-- ghci> refGet6 ('1', '2', '3', '4', '5', '6') 6
+-- Nothing